The invention relates to a process for reducing monomeric aziridines in a polyaziridine reaction by adding carbodiimides as a scavenger, the products obtained by that process and coating compositions prepared from these products.
Polyfunctional aziridines have been shown to be useful as crosslinking agents in various types of waterborne and solventborne coating systems such as carboxylated acrylics, vinyl-acetate, carboxylated urethanes such as polyurethane dispersions (PUDs), styrene acrylics or mixtures thereof.
One important subclass of commercially available polyfunctional aziridines includes reaction products of ethylene imine (EI, aziridine) or propylene imine (PI, methyl aziridine) with trimethylol propane triacrylate (described for example in U.S. Pat. No. 2,596,299 to Bastian). Other commercially important polyfunctional aziridines can be prepared from ethylene imine or propylene imine and pentaerythritol triacrylate. Other polyfunctional aziridines are based on alkoxylated polyols.
Another method of preparing polyfunctional aziridines includes the transesterification of methyl (1-aziridinyl)propionates with polyols catalyzed with tertiary amines (as described in DE 2334656 to Miksovsky), whereby the methyl (1-aziridinyl)propionates are prepared from monomeric aziridines.
Ethylene imine and propylene imine are volatile low molecular weight toxic compounds which are undesired residuals in the processes to form polyfunctional aziridines. The residual monomeric aziridine compound has to be removed sometimes tediously by elaborate distillation methods or lengthy aging processes. This is especially true, if the reaction temperature is low, which is sometimes necessary to avoid discoloration or viscosity problems. Sometimes the residue can exceed 1000 ppm in crude reaction mixtures. It is very desirable to reduce this amount for a commercial product below 500 ppm and even more preferred to reduce it below 100 ppm or ultimately below 50 ppm.
To drive the Michael-type addition reaction of the aziridine and the acrylate to completion, it is possible to use an excess of aziridine. A disadvantage of this approach is the amount of aziridine to be removed from the reaction product under vacuum. With a simple distillation column and a vacuum of about 50 mm Hg it is usually not possible to remove the aziridine level to below 300 ppm on a commercial time scale, for example in a day. To achieve an aziridine level below that, it is necessary to use refined equipment, like an expensive falling-film or wiping-film evaporator, or longer distillation times which is economically unfavorable. Another method to drive the Michael addition uses an excess of acrylate. The residual aziridine levels can thereby be reduced to less than 50 ppm, which sometimes require considerable aging times up to several months, which again is economically unfavorable.
It is an object of the present invention to provide an easy process for making polyfunctional aziridines without advanced and expensive distillation or purification steps. It is another object of the invention to produce polyfunctional polyaziridines that are rheologically stable and are low in color. It is another object of the invention to develop a process that results in low monomeric aziridine without incurring elaborate vacuum steps or long batch or aging times
The present invention uses a scavenger. The reactions between electrophiles and aziridines have been described in detail in the literature. Copending application Ser. No. 10/137,805 describes an isocyanate as a scavenger. One example of a suitable electrophile is a carbodiimide.
The reaction between carbodiimides and aziridines is known. Baeg et al. disclose a regiospecific palladium-catalyzed cycloaddition of N-substituted aziridines and carbodiimides in the J. Org. Chem. (1992), 57(1), 157-62.
None of the above disclosures however describes the present invention.